The production of dense Si.sub.3 N.sub.4 materials (with densities of .gtoreq.96% of the theoretical density) requires the addition of suitable additives which contribute towards the formation of a liquid phase during the heat treatment to compact the powder molding (sintering under atmospheric or elevated gas pressure, hot pressing, hot isostatic pressing) and, hence, enable compaction to be achieved by the known mechanisms of liquid-phase sintering. Additives of the type in question are oxides or oxide mixtures which, with the oxygen (expressed as SiO.sub.2) always present in Si.sub.3 N.sub.4 powders, form a stable silicate or oxynitride melt phase, which thoroughly wets Si.sub.3 N.sub.4 and in which Si.sub.3 N.sub.4 is soluble to a certain extent, at the temperatures typically applied for sintering. In addition to the oxides, nitrides, carbides, borides or silicides are also used, on the one hand to increase the nitrogen content of the liquid phase, which is generally thought to afford advantages in regard to the high-temperature properties, and on the other hand with a view to positively influencing the mechanical properties, preferably at room temperature, by incorporation of other crystalline secondary phases in the Si.sub.3 N.sub.4 matrix material.
A common sintering additive is MgO both on its own and in combination with other additives or in the form of a compound (DE-A 2 855 859, DE-A 2 353 093, DE-A 2 302 438, DE-A 2 945 146). Hitherto, industrial significance has only been attributed to incorporation of the additives in powder form. Although incorporation or application by precipitation is mentioned in the literature, neither has hitherto resulted in demonstrable advantages in regard to material properties and/or process costs.
However, the use of powder-form MgO or MgO compounds also involves difficulties, above all where water has to be used with a view to obtaining moldable granules by spray drying or with a view to molding by slip casting.
The difficulties are as follows:
MgO hydrates and hydrolyses in aqueous suspensions during processing which, in the case of spray drying, leads to very hard granules which cannot be completely destroyed during press molding and, hence, form strength-limiting defects in the sintered parts. PA1 In view of the high isoelectric point of MgO (pH&gt;11), very high pH values of &gt;11 have to be applied to avoid coagulation during the introduction of MgO into Si.sub.3 N.sub.4 -based slips due to different surface charge characteristics. This leads to numerous technical difficulties, including inter alia serious corrosion of plaster molds. PA1 The hydrolysis of MgO leads to a considerable thickening of the slip, so that processing by slip casting is no longer possible.
In principle, the problem can be avoided by using non-aqueous suspension media, although this does result in very serious technical complications and requires elaborate processing measures. To overcome these problems, the MgO is often introduced in the form of a compound, for example Mg spinel, MgAl.sub.2 O.sub.4 or cordierite 2 MgO.2 Al.sub.2 O.sub.3.5 SiO.sub.2, rather than as pure oxide. Apart from only gradual improvements in regard to the pH value, the isoelectric point and the tendency towards hydration, this process has the disadvantage that the ratio of MgO to residual MeO is very limited.
In addition, the strength level of the sintered materials obtained by this process does not always meet the relatively stringent demands which such materials are expected to satisfy.
Accordingly, the problem addressed by the present invention was to provide a material which would satisfy these requirements.